This invention relates to a method of producing silicon. The element silicon is being used in considerable quantities in metallurgy as an alloy additive and in its pure form is being used increasingly in the semiconductor industry. The need for silicon for producing solar cells in order to utilize the sun's energy directly will rise sharply in the future.
After oxygen, silicon is the most commonly occurring element in the Earth's crust. It is available almost everywhere--combined in quartz and silicates, but also in many other minerals.
Technically silicon is based on quartz sand which can be reduced to silicon by coal or metals such as aluminium, magnesium or sodium. This raw silicon is purified for the semiconductor industry and is converted into SiCl.sub.4 at an elevated temperature, for example, with the aid of chlorine gas. SiCl.sub.4 allows for further purification by means of fractional distillation.
Elemental silicon can be obtained in a very pure state in polycrystalline form, say on a hot silicon rod, by means of pyrolysis, i.e. the breakdown of a mixture for example of H.sub.2 and SiCl.sub.4. Due to the so-called "pulling in regions" or "crucible pulling", the polycrystalline silicon can be converted into monocrystalline rods. "Doped" monocrystalline rods of silicon, i.e., provided with intentional impurities, are chopped up into wafers. These wafers are the most important starting material for the semiconductor industry. Polycrystalline silicon wafers can also be used for silicon solar cells and their manufacturing costs are substantially lower than the manufacturing costs of monocrystalline wafers.
Even lower manufacturing costs can be expected for silicon solar cells in which only a thin film of silicon is applied to a suitable carrier, say by means of pyrolytic breakdown of a suitable silicon compound such as SiH.sub.4, for example.
All of the known methods of manufacturing silicon, silicon wafers or silicon layers have two disadvantages particularly with respect to manufacturing cheap solar cells with a high degree of efficiency:
1. Sillicon wafers or layers made from sufficiently pure silicon with a good crystal quality are still relatively expensive; and PA1 2. Sufficiently cheap wafers or layers comprising silicon are usually too poor in quality for good solar cells.
These disadvantages arise from the fact that, on the one hand, too many processing sequences are required to produce the silicon wafer or layer after the silicon has been reduced. On the other hand, the processes of purifying and reducing the polycrystalline silicon are relatively expensive. Moreover, fluid silicon is a material which requires a large outlay of energy and material and which is contaminated by almost all materials with which it comes into contact mechanically at high temperature (e.g. the crucible or the material of the substrate).